Mike Brown is an astronomer, specifically one who studies Kuiper Belt Objects, those giant frozen iceballs that haunt the solar system out past Neptune.

In fact, Neptune’s biggest moon Triton has a lot of characteristics similar KBOs — it may be one captured by Neptune — so observing it gives an interesting opportunity for a compare-and-contrast study. So this past weekend Mike was using the Keck telescope in Hawaii to observe Triton along with its (adoptive?) parent planet, and took this fantastic image of the pair:

[Click to poseidenate.]

This false-color image shows the two worlds in the infrared, specifically at a wavelength of about 1.5 microns, twice what the human eye can see. Methane strongly absorbs this color of light, so where Neptune (in the upper left) looks dark you’re seeing lots of methane clouds, and where it’s bright there are clouds higher up, above the methane. Triton is in the lower right, and is bright because it’s covered in ice which is highly reflective.

Now this is all very pretty and interesting and sciencey, but if you know me at all you know there’s more to this story.

Mike tweeted about the image, and I oohed and ahhhed at it, of course. But then he tweeted again, saying he was also observing Jupiter’s moon Europa, but it was too bright to get good images using the monster 10-meter Keck telescope. It "saturated the detector" which is astronomer-speak for "overexposed".

That’s funny, I thought. Neptune looks fine in the image, and the random noisy grain in it makes it clear Mike wasn’t anywhere near saturating the image. Now I know Europa is closer to the Earth, so it should look brighter, but geez, it’s a moon, and a lot smaller than Neptune. How could it be too bright to image?

It turns out my all–too–human and all–too–miserable sense of scale has failed me again. Math to the rescue!

These four images show Neptune using blue, red, and near-infrared filters. The atmosphere of the planet is laced with methane, which strongly absorbs red light and makes Neptune appear very blue to the eye. I’ve seen it through a telescope and the color is striking; I was using a 25 cm telescope at the time and the planet was just barely resolved as a disk. At 4.5 billion kilometers (3 billion miles) away, it’s amazing even that was discernible!

Today is special: it is now one full Neptunian year since this giant planet was discovered in 1846!

So today is Neptune’s birthday! Um. Well, kinda.

Yeah, as usual, stuff like this gets complicated. I realized this anniversary was coming up about a year ago, and contacted an old friend about it: Kelly Beatty, editor at Sky and Telescope magazine, who then contacted astronomers John Westfall and Roger Sinnott. We had some fun email exchanges about all this! I think I have a good grip on this now, so let me explain.

The short form

First, to celebrate a birthday, you need the birthdate. That’s the first complication. Neptune was discovered on September 23, 1846 by astronomer Johann Galle using star charts by Johann Encke, and they are generally given credit for it. However, that date of September 23 is a bit dicey! Galle and Encke report that they found Neptune on 9/23 at 12:00:15 "Berlin M.T.", according to Westfall. But they reckoned the day starting at noon! And since they’re using Berlin mean time, you have to account for the longitude of Berlin with respect to 0° longitude on Earth. According to Westfall, once you do all that, you get a discovery time of September 23 at 23:06:40.

Worse yet, there may be some imprecision in the exact time the astronomers reported the discovery, although Galle reported the time to a fraction of a second. Westfall reports that might be as much as 1.2 hours, preferring a discovery time of September 24, 1846 at 00:15 GMT.

Who’s right? Turns out, it doesn’t matter much, since we only need to know the time to within a few hours to get the right date for the birthday. Still, Westfall appears to have looked at this pretty hard, so you know what? Good enough. I’ll use his numbers.

A year by any other name would take as long

OK, so we have the birthdate. Now, how long is a Neptune year?

Yeah, well, that turns out not to be so easy to answer either! Read More

Man, is that way out in the black. The probe is now closer to the orbit of Uranus than it is to Saturn, though both planets are over a billion kilometers away from New Horizons right now.

The solar system is frakkin’ BIG (if I may mix my colorful scifi metaphors). If you’re still not sure just how roomy things are out there, even at its current speed of 16.5 km/sec (10 miles/sec) — fast enough to cross the entire United States in five minutes — New Horizons won’t pass the orbit of Uranus until March 18, 2011, more than a year from now. Neptune’s orbit isn’t until August 24, 2014.

One thing to notice: from this point of view, planets revolve around the Sun in a counterclockwise fashion. Given the position of Pluto, you can see the two are heading for a close encounter soon. Well, for a sufficiently broad definition of "soon": July 14, 2015.